Stellar classification
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature. Morgan-Keenan system The modern classification system is known as the Morgan–Keenan (MK) classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals, forming the star's spectral type. Most stars are currently classified under the Morgan-Keenan (MK) system using the letters O'', ''B, A'', ''F, G'', ''K, and M'', a sequence from the hottest (''O type) to the coolest (M'' type). Each letter class is then subdivided using a numeric digit with ''0 being hottest and 9'' being coolest (e.g. A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class ''D for white dwarfs and classes S'' and ''C for carbon stars. In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0'' or ''Ia+ is used for hypergiants, class I'' for ''supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V'' for ''main-sequence stars, class sd (or VI) for sub-dwarfs, and class D'' (or ''VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K. The spectral classes O through M, as well as other more specialized classes, are subdivided by Arabic numerals (0–9), where 0 denotes the hottest stars of a given class. For example, A0 denotes the hottest stars in class A and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2. Conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A class star, which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body. Harvard spectral classification The older Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions. Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest. Cooler than M star classes The new spectral types L, T, and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and brown dwarfs that are very faint in the visible spectrum. Brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types. Brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes, faster the less massive they are; the highest-mass brown dwarfs cannot have cooled to Y or even T dwarfs within the age of the universe. Because this leads to an unresolvable overlap between spectral types effective temperature and luminosity for some masses and ages of different L-T-Y types, no distinct temperature or luminosity values can be given. Class L Class L dwarfs get their designation because they are cooler than M stars and L is the remaining letter alphabetically closest to M. Some of these objects have masses large enough to support hydrogen fusion and are therefore stars, but most are of substellar mass and are therefore brown dwarfs. They are a very dark red in color and brightest in infrared. Their atmosphere is cool enough to allow metal hydrides and alkali metals to be prominent in their spectra. Due to low surface gravity in giant stars, TiO- and VO-bearing condensates never form. Thus, L-type stars larger than dwarfs can never form in an isolated environment. It may be possible for these L-type supergiants to form through stellar collisions, however. An example of which is V838 Monocerotis while in the height of its luminous red nova eruption. Class T Class T are methane dwarfs, cool brown dwarfs with surface temperatures between approximately . Their emission peaks in the infrared. Methane is prominent in their spectra. Classes T and L could be more common than all the other classes combined if recent research is accurate. Because brown dwarfs persist for so long—a few times the age of the universe—in the absence of catastrophic collisions these smaller bodies can only increase in number. Study of the number of proplyds (protoplanetary disks, clumps of gas in nebulae from which stars and planetary systems are formed) indicates that the number of stars in the galaxy should be several orders of magnitude higher than what was previously conjectured. It is theorized that these proplyds are in a race with each other. The first one to form will become a protostar, which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main-sequence stars or brown dwarfs of the L and T classes, which are quite invisible to us. Class Y Brown dwarfs of spectral class Y are cooler than those of spectral class T and have qualitatively different spectra from them. A total of 17 objects have been placed in class Y as of August 2013. Although such dwarfs have been modelledY-Spectral class for Ultra-Cool Dwarfs, N.R.Deacon and N.C.Hambly, 2006 and detected within forty light-years by the Wide-field Infrared Survey Explorer (WISE)Stars as Cool as the Human Body NASA spots chilled-out stars cooler than the human bodyNASA'S Wise Mission Discovers Coolest Class of Stars there is no well-defined spectral sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2. The spectra of these prospective Y objects display absorption around 1.55 micrometers. Delorme et al. have suggested that this feature is due to absorption from ammonia, and that this should be taken as the indicative feature for the T-Y transition. In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class. However, this feature is difficult to distinguish from absorption by water and methane, and other authors have stated that the assignment of class Y0 is premature. The latest brown dwarf proposed for the Y spectral type, WISE 1828+2650, is a > Y2 dwarf with an effective temperature originally estimated around 300 K, the temperature of the human body.European Southern Observatory. "A Very Cool Pair of Brown Dwarfs", 23 March 2011 Parallax measurements have, however, since shown that its luminosity is inconsistent with it being colder than ~400 K. The coolest Y dwarf currently known is WISE 0855−0714 with an approximate temperature of 250 K. The mass range for Y dwarfs is 9–25 Jupiter masses, but young objects might reach below one Jupiter mass, which means that Y class objects straddle the 13 Jupiter mass deuterium-fusion limit that marks the current IAU division between brown dwarfs and planets. Carbon star classifications Carbon-stars are stars whose spectra indicate production of carbon—a byproduct of triple-alpha helium fusion. With increased carbon abundance, and some parallel s-process heavy element production, the spectra of these stars become increasingly deviant from the usual late spectral classes G, K, and M. Equivalent classes for carbon-rich stars are S and C. The giants among those stars are presumed to produce this carbon themselves, but some stars in this class are double stars, whose odd atmosphere is suspected of having been transferred from a companion that is now a white dwarf, when the companion was a carbon-star. Class C Originally classified as R and N stars, these are also known as carbon stars. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C with N0 starting at roughly C6. Another subset of cool carbon stars are the C-J type stars, which are characterized by the strong presence of molecules of 13CN in addition to those of 12CN.Bouigue, R. (1954). Annales d'Astrophysique, Vol. 17, p. 104 A few main-sequence carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants. There are several subclasses: * C-R – Formerly its own class (R'') representing the carbon star equivalent of late G to early K-type stars. * C-N – Formerly its own class representing the carbon star equivalent of late K to M-type stars. * C-J – A subtype of cool C stars with a high content of 13C. * C-H – Population II analogues of the C-R stars. * C-Hd – Hydrogen-deficient carbon stars, similar to late G supergiants with CH and C2 bands added. Class S Class S stars form a continuum between class M stars and carbon stars. Those most similar to class M stars have strong ZrO absorption bands analogous to the TiO bands of class M stars, whereas those most similar to carbon stars have strong sodium D lines and weak C2 bands.Keenan, P. C. (1954). ''Astrophysical Journal, vol. 120, p. 484 Class S stars have excess amounts of zirconium and other elements produced by the s-process, and have more similar carbon and oxygen abundances than class M or carbon stars. Like carbon stars, nearly all known class S stars are asymptotic-giant-branch stars. The spectral type is formed by the letter S and a number between zero and ten. This number corresponds to the temperature of the star and approximately follows the temperature scale used for class M giants. The most common types are S3 to S5. The non-standard designation S10 has only been used for the star Chi Cygni when at an extreme minimum. The basic classification is usually followed by an abundance indication, following one of several schemes: S2,5; S2/5; S2 Zr4 Ti2; or S2*5. A number following a comma is a scale between 1 and 9 based on the ratio of ZrO and TiO. A number following a slash is a more recent but less common scheme designed to represent the ratio of carbon to oxygen on a scale of 1 to 10, where a 0 would be an MS star. Intensities of zirconium and titanium may be indicated explicitly. Also occasionally seen is a number following an asterisk, which represents the strength of the ZrO bands on a scale from 1 to 5. Classes MS and SC Classes MS and SC are intermediary carbon-related classes. In between the M and S classes, border cases are named MS stars. In a similar way, border cases between the S and C-N classes are named SC or CS. The sequence M → MS → S → SC → C-N is hypothesized to be a sequence of increased carbon abundance with age for carbon stars in the asymptotic giant branch. Wolf–Rayet classifications Class W or WR represents the Wolf–Rayet stars, notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by stellar winds, thereby directly exposing their hot helium shells. Class W is further divided into subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers). White dwarf classifications The class D (for Degenerate) is the modern classification used for white dwarfs - low-mass stars that are no longer undergoing nuclear fusion and have shrunk to planetary size, slowly cooling down. Class D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere. The white dwarf types are as follows: * DA – a hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines. * DB – a helium-rich atmosphere, indicated by neutral helium, He I, spectral lines. * DO – a helium-rich atmosphere, indicated by ionized helium, He II, spectral lines. * DQ – a carbon-rich atmosphere, indicated by atomic or molecular carbon lines. * DZ – a metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM). * DC – no strong spectral lines indicating one of the above categories. * DX – spectral lines are insufficiently clear to classify into one of the above categories. The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/''T''eff, where T''eff is the effective surface temperature, measured in kelvins. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9. Two or more of the type letters may be used to indicate a white dwarf that displays more than one of the spectral features above. '''Extended white dwarf spectral types:' * DAB – a hydrogen- and helium-rich white dwarf displaying neutral helium lines. * DAO – a hydrogen- and helium-rich white dwarf displaying ionized helium lines. * DAZ – a hydrogen-rich metallic white dwarf. * DBZ – a helium-rich metallic white dwarf. A different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars: References Category:Star classification system